This invention relates to liquid crystalline electro-optic cells and, more particularly, to contrast enhancement of such cells employing an alignment film provided in accordance with the Janning technique.
The Janning technique, generally speaking, comprises vacuum depositing and alignment materials such as platinum, gold, tin, lead, aluminum, copper, silver, silicon monoxide, or chromium onto a supportive substrate at a chosen angle (typically about 80.degree.) to the normal to the substrate. In this manner, an alignment film is created having film growth oriented in a direction which makes an acute angle to the substrate. See, for example, U.S. Pat. Nos. 3,834,792 and 3,944,331, both to Janning, hereby expressly incorporated by reference. As stated in these patents, the alignment film aligns liquid crystal molecules of a liquid crystalline material when the liquid crystalline material is placed on the alignment film.
These alignment films can be used in accordance with the aforementioned U.S. Pat. No. 3,944,331 to provide a nematic liquid crystalline material in a twisted nematic structure. A typical resulting cell has the nematic liquid crystalline material sandwiched between two electroded supporting substrates and in contact with an aligning film on the inner surface of each substrate. The alignment film coated substrates are then oriented so that the alignment direction of the nematic liquid crystalline material in contact with one alignment film is substantially orthogonal to the alignment direction of the nematic liquid crystalline material in contact with the other alignment film. Since the molecules of the nematic liquid crystalline material in contact with each of the alignment films is oriented by the films in directions substantially orthogonal to one another the molecules of the nematic liquid crystalline material in contact with one alignment film are rotated substantially 90.degree. from the molecular alignment of the molecules in contact with the other alignment film. In this manner, the twisted nematic structure is provided.
It has been found that the alignment film is itself birefringent, produces a conoscopic pattern, and depolarizes off axis light. This birefringence is additive to that of the liquid crystalline material. However, when the liquid crystal material is rendered homeotropic by application of an electrical field (the helical orientation of the liquid crystalline material is broken and the molecules are aligned substantially parallel to one another due to the electrical field) the birefringence remains, limiting contrast and is therefore an undesirable effect.
Accordingly, it is desirable to provide a solution which neutralizes or compensates for this birefringence of the alignment film.